[pseudo-gen] Add -o flag, make --grammar required

Virtually all LLVM tools accept a `-o` flag, so add one. This will make it
possible to possibly add a --write-if-changed flag later. It also makes it
so that the file isn't partially written if the tool oesn't run successfully.

Marking --grammar as `Required` allows removing some manual
verification code for it.

Differential Revision: https://reviews.llvm.org/D126373